(1) Field of the Invention
The present invention relates to aluminum heat exchangers, and in particular, to a method for providing the heat exchanger of a serpentine type.
(2) Description of the Prior Art
Heat exchangers of the serpentine type have been used for, for example, a refrigerant evaporator in an automotive air conditioning system, as shown in, for example, U.S. Pat. Nos. 4,350,025, and 4,353,224.
The serpentine-type heat exchanger comprises a flat metal tube having a refrigerant passageway or parallel passageways therein extending in a longitudinal direction of the tube. The flat tube is bent to weave up and down, or formed in a serpentine-anfractuous shape, and therefore, has a plurality of parallel portions spaced apart from one another and a plurality of U-shaped curved portions connecting adjacent ones of the parallel portions, respectively. A plurality of corrugated fin units are disposed in spaces between adjacent ones of the parallel portion of the tube and are joined thereto by brazing. Each of the corrugated fin units is formed by bending a thin plate in a corrugated form so that a number of crests are formed in opposite side surfaces of the unit alternatively. The crests in the opposite sides of the unit are joined by brazing to flat side surfaces of the opposite parallel portions of the tube.
As high heat-conductivity materials for the flat tube and the fin units, aluminum metals including aluminum and aluminum alloy are usually used. Such heat exchangers using aluminum metals are referred to as aluminum heat exchanger.
In a known serpentine-type aluminum heat exchanger, the serpentine-anfractuous flat tube is usually made of an aluminum metal having 99 wt. % or more Al, for example, AA 1050 (which comprises, by weight, 0.25% or less Si, 0.40% or less Fe, 0.05% or less Cu, 0.05% or less Mn, 0.05% or less Mg, 0.05% or less Zn, 0.03% or less Ti and 99.50% or more Al). While, an aluminum alloy brazing sheet is used for preparing the corrugated fin unit member, which has a core metal of AA 3003 (which comprises, by weight, 0.6% or less Si, 0.7% or less Fe, 0.05-0.20% Cu, 1.0-1.5% Mn, 0.10% or less Zn and the balance Al) with a cladding of an aluminum alloy brazing filler metal, such as AA 4343, 4045 or 4047 (which comprises, by weight, 0.30% or less Cu, 5-13% Si, 0.8% or less Fe, 0.15% or less Mn, up to 0.1 % Mg, 0.20% or less Zn, up to 0.20% Ti, and the balance substantially Al). The brazing sheet is formed in a form of the corrugated fin unit, and the fin unit members thus formed are disposed in spaces between adjacent ones of parallel portions of the flat tube so that the crests in the opposite sides of each fin unit member are in contact with the opposite parallel portions of the flat tube. Then, the flat tube and fin unit members are heated in the assembled relation to a brazing temperature of about 600.degree. C., and are joined by brazing.
In the known serpentine-type aluminum heat exchanger, the flat tube tends to suffer from pittings by corrosion because the aluminum alloy AA 1050 of the flat tube is baser in the corrosion potential than the aluminum alloy AA 3003 of the fin unit material. However, use of another aluminum metal having a corrosion potential equal to, or baser than, that of the flat tube for the core metal of the brazing sheet results in deformation of the fin units during the brazing operation, because elements of the aluminum alloy brazing filler metal diffuse into the core alloy during the brazing operation to lower the melting point of the core metal. Further, the core metal becomes nobler than the flat tube as another result of the diffusion, so that the flat tube still tends to suffer from the pittings.
Moreover, the use of the brazing sheet results in high cost of the heat exchanger.
Furthermore, in the known serpentine-type aluminum heat exchanger, the fin unit has a coating of the aluminum alloy brazing metal layer which is lower in the heat conductivity than the core metal and the flat tube. This means that the aluminum alloy brazing metal layer on the fin unit degrades the heat exchanging property of the exchanger.
In order to dissolve such problems, a novel serpentine-type aluminum heat exchanger and a method for producing the same are proposed in a copending U.S. application Ser. No. 644,816 filed Aug. 27, 1984 in the name of Hisa Aoki which application is assigned to the same assignee.
The novel serpentine-type aluminum heat exchanger comprises a serpentine-anfractuous flat tube of an aluminum alloy and a plurality of corrugated fin units made of an aluminum alloy having a high aluminum content of 99 wt. % of more and joined to the flat tube by brazing metal coating layers fixed onto flat surfaces of parallel portions of the serpentine-anfractuous flat tube.
The novel exchanger is produced by preparing the serpentine-anfractuous flat tube of an aluminum alloy, the corrugated fin units and foil plates of an aluminum alloy brazing filler metal, disposing the fin units in spaces between adjacent ones of parallel portions of the serpentine-anfractuous flat tube with foil plates being interposed between respective fin units and opposite parallel portions of the flat tube, and heating the flat tube, the fin units, and the foil plates in the assembled relation to the brazing temperature.
In the novel aluminum heat exchanger, the flat tube is protected from pittings due to the difference between the corrosion potentials of the flat tube and the fin units, because the flat tube is substantially nobler in the corrosion potential than that of the fin units and because the surface of the flat tube is coated with the aluminum brazing metal layer. Further, since the aluminum metal of the fin unit is excellent in the heat conductivity, the heat exchanging property is improved in comparison with the known aluminum heat exchanger.
However, in the method proposed in the aforementioned copending U.S. patent application Ser. No. 644,816, foil plates of aluminum alloy brazing filler metal are merely interposed between each fin unit and the opposite parallel portions of flat tube. Therefore, it is difficult to maintain foil plates stable in their proper places during a period from the assembling process to the brazing process, that is, the foil plates may fall out from the proper place. Accordingly, the proposed method has a problem.